During the previous funding period, we utilized our in vitro and in vivo models of the multiple myeloma (MM) cell in the bone marrow (BM) milieu to demonstrate the molecular mechanisms whereby proteasome inhibitors target tumor cells, host tumor interactions, and the BM microenvironment to overcome conventional drug resistance. We then rapidly translated these laboratory findings to the clinic leading to the FDA approval of Bortezomib for relapsed refractory MM in 2003 and relapsed MM in 2005. Importantly, our correlative science studies of patient samples have identified genomic profiles of patients most likely to respond. Although Bortezomib represents a major advance, not all patients respond, those that respond relapse, and no patients are cured. This proposal will therefore focus on using oncogenomics to define mechanisms of Bortezomib resistance. We will then validate novel targeted therapies to overcome Bortezomib resistance for evaluation in derived clinical trials.. To achieve these goals, we will pursue the following Specific Aims: to define the molecular determinants of proteasome inhibitor resistance using high throughput RNAi screens interrogating the effects of silencing the "druggable genome" in a proteasome inhibitor resistant MM cell line (Specific Aim 1);to define strategies to overcome proteasome inhibitor resistance using in vitro preclinical models (Specific Aim 2);and to conduct preclinical in vivo studies and clinical trials to enhance sensitivity or overcome resistance to proteasome inhibitors (Specific Aim 3). This new paradigm to overcome proteasome inhibitor resistance in MM has great promise not only to change the natural history of MM, but also to serve as a model for targeted therapeutics directed to improve outcome of patients with other hematologic malignancies as well as solid tumors.